For the purpose of removing substances usually eliminated with urine and for the purpose of withdrawing fluid, use is made of various methods for machine-aided blood cleaning or blood treatment in acute or chronic kidney failure. Diffusive substance transport predominates in the case of haemodialysis (HD), whilst convective substance transport via the membrane takes place in the case of haemofiltration (HF). Haemodiafiltration (HDF) is a combination of the two methods.
During extracorporeal blood treatment, the patient's blood flows via an arterial branch of a tube-line system into a blood treatment apparatus, for example a haemodialyser or haemofilter, and flows from the blood treatment apparatus via a venous branch of the line system back to the patient. The blood is conveyed by means of a blood pump, in particular a roller pump, which is arranged in the arterial branch of the line system. Fluid can be withdrawn from the patient during the extracorporeal treatment (ultrafiltration).
One of the main complications with extracorporeal blood treatment is an acute drop in blood pressure (hypotony), which can be caused by an excessively high or rapid fluid withdrawal. There are various solutions to this problem. On the one hand, blood pressure monitors are known which continuously monitor a change in the blood pressure and regulate the ultrafiltration in dependence on the change in blood pressure. On the other hand, blood volume monitors are known which measure the relative blood volume during the dialysis treatment and carry out a regulation of the ultrafiltration in dependence on the relative blood volume.
DE-C-197 46 377 describes a device for measuring blood pressure, which is based on the detection of the propagation rate or transit time of the pulse waves being propagated via the arterial vessel system of the patient, said pulse waves being generated by the patient's heart contractions.
There is known from DE-A-40 24 434 a device for the regulation of ultrafiltration, in which the pressure in the extracorporeal circuit is monitored in order to determine the relative blood volume. The change in the blood volume is deduced from the change in the pressure in the course of the blood treatment compared with the pressure at the start of the treatment.
There is known from DE 100 51 943 A1 a method for the non-invasive blood pressure measurement of patients on the basis of pulse-wave transit times during an extracorporeal blood treatment.